Hyperspectral camera

ABSTRACT

A camera includes a first lens configured to focus incoming light onto a reflective slit assembly. The reflective slit assembly comprises an elongated strip of reflective material configured to reflect some but not all of the incoming light as return light. The first lens is configured to at least partially collimate the return light from the elongated strip of reflective material. A first mirror is configured to reflect the return light from the first lens. A second mirror is configured to reflect the return light from the first mirror. An optical element is configured to separate the return light from the first mirror as a function of wavelength. A second lens is configured to focus the return light from the optical element onto a first detector. The first detector is configured to measure intensities of the return light as a function of two dimensional position on the first detector.

FIELD OF THE INVENTION

The present invention relates to spectral imaging, and more particularlyto a compact optical system for push broom hyperspectral imaging.

BACKGROUND OF THE INVENTION

Hyperspectral imaging involves collecting spectral data for each pixelin the image of a scene. The spectral data, in combination with thespatial location (i.e., the pixel) from which it originates, can be usedto find objects, identify materials, or detecting processes. There arethree general types of spectral imagers. There are push broom (line)scanners and whisk broom (point) scanners which involve spatial scanningover time, band sequential scanners which involve spectral scanning thatacquire images of an area at different wavelengths, and snapshothyperspectral imaging which uses an array to generate an image in aninstant. Hyperspectral imaging look at objects using a vast portion ofthe electromagnetic spectrum. Certain objects leave unique‘fingerprints’ in the electromagnetic spectrum, especially when combinedwith the spatial location within the image. Known as spectralsignatures, these ‘fingerprints’ enable identification of the materialsor objections that make up a scanned scene.

Push broom hyperspectral imaging involves capturing a strip of the sceneand spectrally dispersing the slit image with a prism or grating tocollect the spectral data from the strip. FIG. 1 conceptionally showsthe push broom imaging concept, where light 1 from a strip 2 of a scene3 is captured by a camera 4. The light 1 in this example originates fromthe entire dimension of the scene 3 in the X direction, and from just anarrow portion of the dimension of the scene 3 in the orthogonal Ydirection. However, it is also possible to conduct multiple scans ofjust part of the scene dimension in the X direction. The light 1 iscollimated by one or more lenses 5, separated by wavelength using one ormore optical elements 6 (e.g., diffraction grating, prism, etc.), andfocused onto a detector 7 by one or more lenses 8, as conceptually shownin FIG. 2 . In order to capture only a strip of light 1 from the sceneat any given time, the light from the scene passes through atransmissive strip (not shown) placed in the optical path between lens 5and detector 7, which only transmits the strip of light 1 portion of allthe light originating from the scene 3. The remaining light from thescene is blocked, scattered or reflected away such that it does notreach the detector 7. The transmissive strip can be, for example, anopaque sheet with an elongated aperture (i.e., a slit) through which thestrip of light 1 passes. The detector 7 measures the amplitudes of thewavelength components of the strip of light 1 for each position acrossthe X direction dimension of the image. This spatial and spectral datafor the strip 2 of the scene 3 can be represented in a hyperspectralimage also referred to as a hyperspectral data cube 9 having two spatialdimensions (X, Y), and one spectral dimension (X), as shown in FIG. 3 .The strips 2 of the scene 3 are scanned separately, and thespatial/spectra data from the strips are stitched together to create afull hyperspectral data cube 9 of the scene 3, as shown in FIG. 4 . Thedata value for each pixel within the cube represents the value of aparticular wavelength detected at one spatial location within the scene3. Conventional push broom hyperspectral cameras are large and bulkybecause of the large number of optical components needed.

There is a need for a push broom hyperspectral imaging device thatutilizes a simpler and more compact optical design.

BRIEF SUMMARY OF THE INVENTION

The aforementioned problems and needs are addressed by a camera thatincludes a first lens configured to focus incoming light onto areflective slit assembly. The reflective slit assembly comprises anelongated strip of reflective material configured to reflect some butnot all of the incoming light as return light. The first lens isconfigured to at least partially collimate the return light from theelongated strip of reflective material. A first mirror is configured toreflect the return light from the first lens. A second mirror isconfigured to reflect the return light from the first mirror. An opticalelement is configured to separate the return light from the first mirroras a function of wavelength. A second lens is configured to focus thereturn light from the optical element onto a first detector. The firstdetector is configured to measure intensities of the return light as afunction of two dimensional position on the first detector.

Other objects and features of the present invention will become apparentby a review of the specification, claims and appended figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a conventional hyperspectral camera.

FIG. 2 is a schematic diagram of a conventional hyperspectral camera.

FIG. 3 is a graphical representation of a single line of pixels within ahyperspectral data cube.

FIG. 4 is a graphical representation of all the pixels within ahyperspectral data cube.

FIG. 5 is a schematic diagram of a camera.

FIG. 6 is a perspective view of the reflective slit.

FIG. 7A is a side view of a diffraction grating as the optical elementthat separates light as a function of wavelength.

FIG. 7B is a side view of a prism as the optical element that separateslight as a function of wavelength.

FIG. 8 is a side cross sectional view of the first or second lens.

FIG. 9 is a schematic diagram of the camera illustrating the path ofincoming light into the camera.

FIG. 10 is a schematic diagram of the camera illustrating the path ofreturn light inside the camera.

FIG. 11 is a schematic diagram illustrating the separation of light as afunction of wavelength within the camera.

FIG. 12 is a perspective view of an alternate example of the reflectiveslit assembly.

FIG. 13 is a perspective view of an alternate example of the reflectiveslit assembly.

FIG. 14 is a schematic diagram of an alternate example of the camera.

FIGS. 15A and 15B are schematic diagrams of alternate examples of thecamera.

FIGS. 16A and 16B are schematic diagrams of alternate examples of thecamera.

FIG. 17 is a side cross sectional view of an alternate example of thefirst or second lens.

DETAILED DESCRIPTION OF THE INVENTION

A push broom hyperspectral imaging device type camera that utilizes asimple and compact optical configuration for capturing spatial andspectral data from strip images of a scene is disclosed. FIG. 5illustrates an example of camera 20, which includes an entrance pupil22, a first mirror 24, a first lens 26, a reflective slit assembly 28, asecond mirror 30, an optical element 32 that separates light as afunction of wavelength, a second lens 34 and a (first) detector 36.

The entrance pupil 22 is optional, and can be an aperture ortransmissive material in an otherwise opaque material through whichlight from the scene passes. First mirror 24 is semitransparent suchthat it allows at least some of the light entering through the entrancepupil 22 to pass (i.e., transmit) through first mirror 24. Anon-limiting example of first mirror 24 is a 50 percent splitter cube orother type of 50 percent beam splitter or mirror that transmits 50percent of the light, and reflects 50 percent of the light, incident onit from either direction. First lens 26 focuses the light from the firstmirror 24 onto reflective slit assembly 28. Reflective slit assembly 28(better shown in FIG. 6 ) has a top surface that is made of, or coveredby, light absorbing or scattering material 38 except for an elongatedstrip of reflective material 40 (also referred to herein as mirror strip40) having a length L in the (first) X direction and a width W in the(second) Y direction (i.e., the (first) X direction is orthogonal to the(second) Y direction), where length L is greater than width W (i.e., thelight absorbing material 38 is immediately adjacent the mirror strip40). Mirror strip 40 is preferably configured to match the fulldimension of the image reaching reflective slit assembly 28 in the Xdirection, but only a small dimension of the image reaching reflectiveslit assembly 28 in the Y direction, which is achieved by having lengthL being greater than width W. The light absorbing or scattering material38 surrounds mirror strip 40 and is positioned to absorb or scatter thelight not being reflected by mirror strip 40. Light absorbing orscattering material 38 can be, for example, metal oxides like blackchromium (chromium oxide), silver oxide (Ag2O), etched electrolessnickel-phosphor, iron-oxide, black matrix, carbon, di-electric coatings,copper selenide (CuSe5), graphene, as well as commercially availableblack absorbing materials like Acktar Black, Vantablack, diamond backADLC, and anodized surfaces. The mirror strip 40 can be, for example,any good light reflecting material, such as TiO₂, SiO₂, Ta₂O₅, Cr, Al,Au, Ag, etc.). As a non-limiting example, width W can be, for example,1.5 μm, and length L can be, for example, 3-5 mm (e.g., similar to Xdirection dimension of detector 36).

First mirror 24 is configured to reflect at least some of the lightreflected by the mirror strip 40 (and passing through first lens 26)toward second mirror 30. Second mirror 30 reflects light from firstmirror 24 toward optical element 32, which separates the incoming lightinto different directions based on wavelength (e.g., through diffractionor refraction). As a non-limiting example, optical element 32 can be atransmission diffraction grating as shown in FIG. 7A. The transmissiondiffraction grating can include a transparent substrate 32 a having aperiodic structure 32 b formed thereon or therein that diffracts thelight passing through it in different angles as a function of thewavelength of the light. The periodic structure 32 b could be, forexample, 500 diffraction lines (i.e., elongated ridges and/or valleys)per millimeter that extend lengthwise in the X direction. As anothernon-limiting example, optical element 32 can be a prism as shown in FIG.7B. The prism can include a transparent substrate 32 c with non-parallelsides 32 d (as viewed in the X direction). Second lens 34 focuses thelight from the optical element 32 onto the detector 36. Detector 36 canbe a two dimensional pixel sensor array (i.e., image sensor) that canresolve the two dimensional image created by the optical elements of thecamera 20 by measuring light intensities as a function of twodimensional position on the array. A non-limiting example of detector 36is one that is 2800 μm square, and has a resolution (i.e. a pixel size)of approximately 1 μm per pixel. The output signals from detector 36 areprovided to a processor 70.

First and second lenses 26/34 can be identical to each other. Onenon-limiting example of each first and second lens 26/34 can be a lensstack with two lens components 42 stacked together so that there arefour aspheric surfaces 44 for each lens 26/34, as shown in FIG. 8 . Morethan two lens components 42 can be stacked together if more than fouraspheric surfaces 44 are desired. The dimensions of the two lenscomponents 42 of each lens 26/34 can vary from each other, and/or can bemade of different materials (as a non-limiting example, two differentpolymer materials such as acrylate-based polymer and epoxy-based polymercan be used for the two lens components 42 respectively for improvedchromatic performance). Lenses 26/34 can be PIM (plastic injectionmolded lens), molded glass, machined and polished glass, combinations ofglass lenses (e.g., achromats), glass replica lenses, wafer-leveloptics, or any combination thereof.

Entrance pupil 22, first mirror 24, first lens 26 and reflective slitassembly 28 are arranged along (i.e., a least a portion of each opticalelement is located on) a first optical axis OA1, and second mirror 30,optical element 32, second lens 34 and detector 36 are arranged along asecond optical axis OA2, as shown in FIGS. 9 and 10 . Preferably, butnot necessarily, optical axes OA1 and OA2 are parallel to each other.Doing so provides the advantage of minimizing the space occupied by theoptical elements (i.e., allow for minimizing the footprint taken by allthe optical elements. It also simplifies the design of the opticalelements (e.g., they can be made on a wafer scale because, for example,the lenses in the wafer would have the same pitch).

In operation, incoming light 50 from the scene being scanned enters thecamera 20 through entrance pupil 22 (if one is used), is transmittedthrough first mirror 24, and is focused by first lens 26 onto reflectiveslit assembly 28, as shown in FIG. 9 . For example, the image of thescene is focused onto the two dimensional area of the reflective slitassembly. Most of the light focused onto the reflective slit assembly 28is absorbed or scattered. However, the portion of the incoming light 50focused onto the mirror strip 40 is reflected as return light 52, asshown in FIG. 10 . The return light 52 (which corresponds to only astrip of the image of the scene) from the mirror strip 40 passes throughfirst lens 26 where it is collimated or at least partially collimated.The return light from the first lens 26 is reflected by first mirror 24toward second mirror 30. The return light from the first mirror 24 isreflected by second mirror 30 toward optical element 32. Optical element32 separates the return light 52 from the first and second mirrors 26/30based upon wavelength in the dimension orthogonal to the lengthwisedirection of mirror strip 40 (i.e., orthogonal to the lengthwisedirection of the strip of the scene from which the light originated),and second lens 34 focuses the return light from the first/secondmirrors 26/30 and optical element 32 onto detector 36. Specifically, theoptical element 32 is configured to separate the return light 52 basedupon wavelength in the Y direction, which is orthogonal to the Xdirection in which the lengthwise direction of mirror strip 40 extends(compare FIGS. 6 and 10 ). Therefore, the optical configuration ofcamera 20 preserves the original spatial location of light within theimage in the X direction (as reflected by the mirror strip 40), whileseparating the color components of the light in the Y direction for anygiven location along the X direction. This result is illustrated in FIG.11 , where for each location along the X direction of the mirror strip40, the blue wavelength components 52 b of the return light 52 aredirected to the upper portions of the detector 36 (relative to the Ydirection), the green wavelength components 52 g of the return light 52are directed to the center portions of the detector 36 (relative to theY direction), and the red wavelength components 52 r of the return light52 are directed to the lower portions of the detector 36 (relative tothe Y direction). Therefore, the portion of the image that reaches thedetector 36 is spatially preserved in the X direction while separated bywavelength in the Y direction. The image is captured by the detector 36by measuring intensities of the light as a function of two dimensionalposition on the detector 36. Preferably, the detector 36 is tilted by atilt angle θ so that the portion of the detector 36 receiving the bluewavelength components 52 b is closer to the second lens 34 than theportion of the detector 36 receiving the red wavelength components 52 r,to accommodate for the different focal lengths for different wavelengthsof light. As a non-limiting example, the tilt angle θ can be 4 degrees.It should also be noted that the angle of mirror 30 can be selected tobest match the diffraction/refraction angle of optical element 32.

As shown in FIGS. 9-11 , only light from a single narrow strip withinthe image of the scene being scanned is directed to the detector 36 atany given time. To scan the entire scene, the camera 20 can be movedrelative to the scene being scanned, or optics can be used to shift theincoming light relative to the optics of the camera, whereby data can besequentially captured for individual strips of the scene being scanned.Camera 20 can include (or be connected to) processor 70 for processingthe signals from detector 36 to, for example, process the spatial andspectral data represented by the signals from detector 36. That dataprocessing can include creating a hyperspectral data cube or otherhyperspectral images that represent the collected data and in turnrepresent the scene being scanned, including piecing together the datacollected from individual strips of the scene being sequentiallyscanned. The data processing can also include comparing the spatial andspectral data to a library of known values in order to identify thescene, one or more objects in the scene, and/or or materials in thescene.

FIG. 12 illustrates an alternate example, where reflective slit assembly28 includes a (second) detector 60 covered by transmissive material 58and mirror strip 40, so that a full image of the scene being focusedonto the reflective slit assembly 28 by the first lens 26 can becaptured and measured (i.e., by measuring intensities of the light as afunction of two dimensional position on the detector 60). The imagedetected by detector 60 would have a thin strip missing, correspondingto the location of mirror strip 40. If desired, that missing strip ofthe image can be filled in by processor 70 either by extrapolating fromthe adjacent data or by using the signals from detector 36 thatrepresent that portion of the image that is missing. FIG. 13 illustratesanother alternate example, where transmissive material 58 is omitted.

Wavelength separation need not occur after the return light 52 isreflected by second mirror 30. For example, as illustrated in FIG. 14(but with simplified representations of the light rays after opticalelement 32), optical element 32 can be disposed between first and secondmirrors 24/30, instead of between second mirror 30 and second lens 34 asshown in FIG. 5 . In this example, the return light 52 from the firstmirror 24 is separated by wavelength before being reflected by secondmirror 30.

FIGS. 15A and 15B illustrate alternate examples, where the first mirror24 is positioned such that the incoming light 50 bypasses first mirror24 in reaching first lens 26 (e.g., the first mirror 24 is notpositioned directly between entrance pupil 22 and first lens 26, so theincoming light can proceed to first lens 26 without encountering firstmirror 24). The first mirror 24 is positioned to reflect all of thereturn light 52 from reflective slit assembly 28. The advantage of thisoptical configuration is that there is no loss of a portion of theincoming light 50 caused by passing through the first mirror 24.Incoming light 50 completely avoids first mirror 24. Further, firstmirror 24 can be a high reflecting element that reflects all (orvirtually all) of the return light 52. The avoidance of light loss forboth the incoming light 50 (by not having to pass through first mirror24) and return light 52 (by avoiding configuring first mirror 24 toreflect only some of the return light 52), will increase the levels oflight reaching detector 36 (i.e., increasing any signal to noise ratio).FIGS. 16A and 16B illustrate the optical configurations of FIGS. 15A and15B (but with simplified representations of the light rays after opticalelement 32), except optical element 32 is disposed between first andsecond mirrors 24/30, instead of between second mirror 30 and secondlens 34.

FIG. 17 illustrates another non-limiting example for first and secondlenses 26/34, which includes first lens component 80 and second lenscomponent 84. The first lens component 80 includes two opposing, convex,spheric surfaces 82. First lens component can be a single element, orcan be two elements glued together as shown in FIG. 17 . As anon-limiting example, first lens component 80 can be formed of typeN-BAK4 glass. The spheric surfaces 82 can be uncoated, or can be coated(as a non-limiting example, the coating can be an acrylic polymer ofapproximately 0.03 mm thickness, which can optimize resolution acrossthe field of view). The second lens component 84 includes two opposingsurface 86 and 88, where surface 86 is concave, spheric and faces thefirst lens component 80, and surface 88 is planar. As a non-limitingexample, second lens component 84 can be formed of type N-SF11 glass.Surfaces 86 and 88 can be uncoated or coated. As a non-limiting example,the total thickness of the first and second lens components 80 and 84can be approximately 10 mm.

Camera 20 has many advantages. The camera includes only two lenses 26/34(where the lens 26 is used bidirectionally to focus light onto thereflective slit assembly 28 and focus light reflected from reflectiveslit assembly 28), two mirrors and a folded design whereby the opticalcomponents are arranged on two parallel optical axes. Thus, the size ofthe camera can be made to be relatively small, to enable the camera 20to be wearable, or integrated into a mobile device such as a cell phone.The camera 20 has no moving parts, simplifying operation, reducing powerconsumption, and providing increased reliability. The optical systemprovides increased resolution compared to other camera systems withlarger and more numerous optical components. The use of reflective slitassembly 28 allows for the inclusion of optional detector 60 positionedalong the first optical axis OA1 without blocking the light reflected bymirror strip 40 to optical element 32 and detector 36. Processor 70 cancombine the data from both detector 36 and detector 60 to create anoverlay image of a regular image (from detector 60) and a hyperspectralimage (from detector 36).

It is to be understood that the present invention is not limited to theexample(s) described above and illustrated herein, but encompasses anyand all variations falling within the scope of any claims. For example,references to the present invention herein are not intended to limit thescope of any claim or claim term, but instead merely make reference toone or more features that may be covered by one or more of the claims.Materials, processes and numerical examples described above areexemplary only, and should not be deemed to limit the claims.

What is claimed is:
 1. A camera, comprising: a first lens configured tofocus incoming light onto a reflective slit assembly, wherein thereflective slit assembly comprises an elongated strip of reflectivematerial configured to reflect some but not all of the incoming light asreturn light; wherein the first lens is configured to at least partiallycollimate the return light from the elongated strip of reflectivematerial; a first mirror configured to reflect the return light from thefirst lens; a second mirror configured to reflect the return light fromthe first mirror; a prism or a grating positioned to separate the returnlight from the first mirror as a function of wavelength; and a secondlens configured to focus the return light from the prism or the gratingonto a first detector, wherein the first detector is configured tomeasure intensities of the return light as a function of two dimensionalposition on the first detector.
 2. The camera of claim 1, furthercomprising: a processor configured to create a hyperspectral image fromthe intensities of light measured by the first detector.
 3. The cameraof claim 1, wherein: the first mirror, the first lens and the reflectiveslit assembly are arranged along a first optical axis; and the secondmirror, the second lens and the first detector are arranged along asecond optical axis that is parallel to the first optical axis.
 4. Thecamera of claim 3, wherein the prism or the grating is arranged alongthe second optical axis.
 5. The camera of claim 1, wherein the prism orthe grating is a transmission diffraction grating.
 6. The camera ofclaim 1, wherein the prism or the grating is a prism.
 7. The camera ofclaim 1, wherein: the elongated strip of reflective material has alength L and a width W; the length L is greater than the width W; thelength L extends in a first (X) direction; and the prism or the gratingis positioned to separate the return light as a function of wavelengthin a second (Y) direction that is orthogonal to the first (X) direction.8. The camera of claim 7, wherein the prism or the grating is a gratingthat includes diffraction lines that extend lengthwise in the first (X)direction.
 9. The camera of claim 1, wherein the prism or the grating isdisposed between the second mirror and the second lens.
 10. The cameraof claim 1, wherein the prism or the grating is disposed between thefirst mirror and the second mirror.
 11. The camera of claim 1, whereinthe first mirror is configured to transmit the incoming light to thefirst lens.
 12. The camera of claim 11, wherein the first mirror is a 50percent beam splitter.
 13. The camera of claim 1, wherein the first lenscomprises a lens stack having at least four aspheric surfaces.
 14. Thecamera of claim 1, wherein the second lens comprises a lens stack havingat least four aspheric surfaces.
 15. The camera of claim 1, wherein thefirst lens comprises: a first lens component that includes two opposingconvex spheric surfaces; and a second lens component a first concavesurface facing the first lens component and a second planar surface. 16.The camera of claim 1, wherein the second lens comprises: a first lenscomponent that includes two opposing convex spheric surfaces; and asecond lens component a first concave surface facing the first lenscomponent and a second planar surface.
 17. The camera of claim 1,wherein the reflective slit assembly comprises light absorbing materialimmediately adjacent the elongated strip of reflective material.
 18. Thecamera of claim 1, wherein the reflective slit assembly furthercomprises: a second detector configured to measure intensities ofincoming light focused by the first lens as a function of twodimensional position on the second detector.
 19. The camera of claim 18,further comprising: a processor configured to create an overlay imagefrom the intensities of light measured by the first detector and fromthe intensities of light measured by the second detector.